Piezoelectric ceramic materials are ceramic materials capable of generating a voltage when mechanical force is applied or producing a mechanical force when a voltage is applied.
The performance of a piezoelectric ceramic material is measured mainly by its electromechanical coupling factor (Kp), mechanical quality factor (Qm), ferroelectric constant (K) and dissipation factor (tan .delta.).
In the production of piezoelectric ceramics, the raw materials are mixed, ball-milled, fired, further ball-milled and then sintered. The sintered body is then polished on both sides, adapted with electrodes and then polarized in a silicon oil bath of about 130.degree. C. by applying a high voltage direct current field (usually from 2.5 to 3.5 kV/mm) for from about 10 to about 30 minutes.
The properties of a piezoelectric ceramic is determined primarily by the composition of the raw materials, the process parameters of the production and the completeness of polarization. The performance of a piezoelectric ceramic will unavoidably deteriorate after a certain period of usage. For example, the Kp value of 0.50 right after polarization will spontaneously decrease to 0.48-0.49. To improve the useful life and the efficiency of the piezoelectric ceramics, it has been tried to adopt more critical process or use higher polarization voltage for the production of the piezoelectric ceramics to increase the initial Kp value. However, critical process is quite costly and higher polarization voltage will adversely affect the structure of the final product. It is desirable to improve the performance of piezoelectric ceramics by other simpler and non-destructive methods.
Japanese Laid-Open Patent Application No. JP63-131407 discloses a process to increase the dielectric constant, resonance frequency and resonance impedance of polarized piezoelectric ceramics by applying a constant temperature heat-treatment at a temperature lower than half of the Curie temperature for about 5-15 hours. The Kp value, however, is decreased.
Deutsche Patent No. DE 1796226 discloses a process to decrease the aging rate of piezoelectric ceramic materials by a heat treatment of repeated heating and cooling cycle. The heating and cooling rate is lower than 30.degree./hr. Although the frequency constant of the piezoelectric ceramics is stabilized, the Kp value is decreased.